Delayed onset of electromyographic activity of vastus medialis obliquus relative to vastus lateralis in subjects with patellofemoral pain syndrome

Patellofemoral Pain Syndrome (PFPS) is a common musculoskeletal complaint. The presence of a delay between vastus medialis obliquus (VMO) and vastus lateralis (VL) muscle onset has been suggested in the literature as a possible cause of PFPS, with poor agreement amongst authors on the value of the delay. In this study we computed the delay in the activation of VMO and VL in 15 PFPS patients and 20 age-matched controls (Ctrls) during the following tasks: sit to stand, stand to sit, squat, step up and step down. Activation instants were detected from surface EMG data by a double-threshold statistical detector. In order to compare the muscle activity throughout the task, we computed the delay between the instants in which the VMO and VL normalised envelopes reached subsequent normalised amplitude levels, until the envelope peak. In all investigated tasks but sit to stand, the onset delay was lower or equal then 0.02. s, without group differences. Similarly, no differences between Ctrls and PFPS timing were found throughout all tasks, until the peak. Our results do not support the hypothesis that an onset delay between VMO and VL can be one of the causes of PFPS. © 2010.